Odd!
Try this:
Insert a DVD. When Media Centre opens, close the program.
Go to 'My Computer' and right-click on the icon for your DVD drive. Select 'Explore'. You should now see the file which contains the movie. Right-click on it and select 'Properties'.
Next to 'Opens with', click 'Change'. If Roxio Cineplayer is in the list, click on it, check that there's a tick in the box labelled 'Always open this type of file with this program' and click 'OK'.
(If Roxio Cineplayer isn't in the list, you'll have to click 'Other', navigate to the location of the executable file for Roxio CinePlayer and then double-click on it).
Click 'Apply', then 'OK'.
Chris